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The most massive black holes in the universe are often encircled by thick doughnut-shaped disks of material. This doughnut material ultimately feeds and nourishes the growing black holes tucked inside. Until recently, some of these doughnuts were too thick for any telescopes to penetrate, but now a team of astronomers have used the NASA NuSTAR and ESA XMM-Newton X-ray observatories to peer inside. They publish their work in Monthly Notices of the Royal Astronomical Society.

"Originally, we thought that some black holes were hidden behind walls or screens of material that could not be seen through," said Andrea Marinucci, the lead author of the new study.

NuSTAR recently looked inside one of the densest doughnuts known around a supermassive black hole. This black hole lies at the centre of a well-studied spiral galaxy called NGC 1068, located 47 million light-years away in the direction of the constellation of Cetus. The observations revealed a clumpy doughnut.

"The rotating material is not a simple doughnut as originally thought, but clumpy," said Marinucci.

Doughnuts around supermassive black holes were first proposed in the mid-80s to explain why some black holes are hidden behind gas and dust, while others are not. The idea is that the orientation of the doughnut relative to Earth affects the way we perceive a black hole and its intense radiation. If the doughnut is viewed edge-on, the black hole is blocked. If the doughnut is viewed face-on, the black hole and its surrounding, blazing materials can be detected. This idea is referred to as the unified model because it neatly joins together the different black hole types, based solely upon orientation.

In the past decade, astronomers have been finding hints that these doughnuts aren't as smoothly shaped as once thought. They are more like defect doughnuts with lumps, which a doughnut shop might throw in the trash.

The new discovery is the first time that this clumpiness has been observed in an ultra-thick doughnut, and supports the idea that this phenomenon may be common. The research is important for understanding the growth and evolution of massive black holes and their host galaxies.

"We don't fully understand why some supermassive black holes are so heavily obscured, or why they are clumpy," said co-author Poshak Gandhi of the University of Southampton in the United Kingdom. "This is a subject of hot research."

Both NuSTAR and XMM-Newton observed the supermassive black hole in NGC 1068 simultaneously on two separate occasions between 2014 and 2015. On one of those occasions, in August, 2015, NuSTAR observed a spike in brightness. NuSTAR observes higher-energy X-rays than XMM-Newton, and those high-energy X-rays can uniquely pierce thick clouds around the black hole. The scientists say that the spike in high-energy X-rays was due to a clearing in the thickness of the material entombing the supermassive black hole.

Hidden lair at the heart of NGC 1068. NGC 1068 can be seen in close-up in this view from NASA's Hubble Space Telescope. Image credit: NASA / JPL-Caltech

"It's like a cloudy day, when the clouds partially move away from the sun to let more light shine through," said Marinucci.

"NGC 1068 is well known to astronomers as the first black hole which gave birth to the unification idea. But it is only with NuSTAR that we now have a direct glimpse of its black hole through such clouds, albeit fleeting, allowing a better test of the unification concept," he said.

The team says that future research will address the question of what causes the unevenness in the doughnut material. The answer could come in many flavours. It's possible that a black hole generates turbulence as it chomps on nearby material. The energy given off by young stars could also stir up turbulence, which would then percolate outward through the doughnut. Or, the clumps may be from material that falls onto the doughnut from outside it.

As galaxies form, material migrates toward the centre, where the density and gravity is greatest. The material tends to fall in clumps, almost like a falling stream of water condensing into droplets as it hits the ground.

"We'd like to figure out if the unevenness of the material is being generated from outside the doughnut, or within it," said Gandhi.

"These coordinated observations with NuSTAR and XMM-Newton show yet again the exciting science possible when these satellites work together," said Daniel Stern, the project scientist for NuSTAR at NASA's Jet Propulsion Laboratory in Pasadena, California.

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Images and captions

NuSTAR's view of NGC 1068. NGC 1068 is shown in visible light and X-rays in this composite image. High-energy X-rays (magenta) captured by NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, are overlaid on visible-light images from both NASA's Hubble Space Telescope and the Sloan Digital Sky Survey. The X-ray light is coming from an active supermassive black hole, also known as a quasar, in the center of the galaxy. This supermassive black hole has been extensively studied due to its relatively close proximity to our galaxy. NGC 1068 is about 47 million light-years away in the constellation Cetus.

The supermassive black hole is also one of the most obscured known, blanketed by thick clouds of gas and dust. NuSTAR's high-energy X-ray view is the first to penetrate the walls of this black hole's hidden lair.

Image credit: NASA / JPL-Caltech / Roma Tre Univ.

Hidden lair at the heart of NGC 1068. NGC 1068 can be seen in close-up in this view from NASA's Hubble Space Telescope. NuSTAR's high-energy X-ray eyes were able to obtain the best view yet into the hidden lair of the galaxy's central, supermassive black hole. This active black hole -- shown as an illustration in the zoomed-in inset -- is one of the most obscured known, meaning that it is surrounded by extremely thick clouds of gas and dust.

The NuSTAR data revealed that the torus of gas and dust surrounding the black hole, also referred to as a doughnut, is more clumpy than previously thought. Doughnuts around active, supermassive black holes were originally proposed in the mid-1980s to be smooth entities. More recently, researchers have been finding that doughnuts are not so smooth but have lumps. NuSTAR's latest finding shows that this is true for even the thickest of doughnuts.

Notes for editors

The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3900 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.